Electrically rechargeable (or secondary) metal-air cells or batteries are very attractive power sources in light of their potential high energy density. In this type of cells the oxidizing reactant (oxygen) which undergoes reduction during discharge is supplied from outside the cell. This reaction of oxygen reduction occurs in the presence of water and gives hydroxide ions (OH−). The oxygen is reduced on the surface of the cathode during discharge. The cathode comprises a material that catalyzes the above-mentioned oxygen reduction. Such catalyst includes, inter alia, silver, platinum, perovskite-type oxides, mixed metal oxides with spinel structure etc.
However, until now this type of cells or batteries has not been satisfactory realized because of the degradation of the catalyst material during recharging.
There are many uses for silver catalysts, including the chemical industry (e.g., in reaction of ethylene oxidation), cells or batteries (both primary and rechargeable) and fuel cells with alkaline electrolyte, oxygen sensors, and electrolysis cells.
Sub-micron and nano-size silver metal powder and silver based bulk alloy catalysts can be produced by different methods, including the Raney method of making a “skeleton” catalyst from Ag—Ca, Ag—Mg, Ag—Al and others alloys, chemical precipitation, leaching of Al from heat treated strips of Al—Ag alloys (see, by way of example, U.S. Pat. No. 5,476,535, which is incorporated herein by reference).
The above methods result in a porous silver agglomerate or cluster of primary particles in a range of sizes from tenths of microns to a few millimeters, consisting of primary particles having an average size from about sub-micron to a few hundred microns.
While all the above-described methods can produce silver powder catalysts having sub-micron or nano-size primary particles, these techniques have been found to have a drawback if the final product is used as a catalyst for air or oxygen electrodes for batteries and fuel cells with alkaline electrolyte. This drawback relates to the phenomenon of silver catalyst dissolution in alkaline electrolyte.
Silver, by itself, has a very low rate of dissolution in alkaline solutions. On the other hand, silver oxides have a much higher dissolution rate. At an anodic oxidation of silver in alkaline solution, the first phase transition at a potential of +0.24V (here and throughout this application the potentials are vs. Hg/HgO reference electrode) is Ag→Ag2O. The next phase transition is Ag2O→AgO at a potential of about +0.5V.
These values of potentials were determined in various classic studies for smooth silver foil (see M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, (1966), p. 393). For fine silver powder and for any kind of nano-sized silver catalyst in particular, the formation of silver oxides starts at much more cathodic potentials. Y. Golin, et al. (Electrochimia, Vol. 18, p. 1223) disclose that Ag2O appears on the surface of ultra-fine silver catalyst at a potential of about +0.1V. This range of potentials corresponds to the open circuit voltage (OCV) of oxygen/air electrodes in alkaline electrolyte. If a silver-catalyzed cathode works as a bi-functional electrode, the potentials in the charge mode of operation could reach +0.4-0.5V and even higher, until the process of oxygen evolution occurs. This means that substantially all the problems of silver catalyst dissolution result from the formation of different types of silver oxides at OCV and anodic polarization, and their subsequent decomposition and precipitation.
While there has been some discussion in the technical literature as to the nature of the dissolution of silver oxide, it is generally agreed that the silver is present in alkaline electrolyte in the form of anions, like Ag(OH)2− or AgO− (H. Fleischer, (ed.), Zinc-Silver Oxide Batteries, J. Wiley (1971)), and has a tendency, during decomposition, to slowly form a finely divided black deposit of metallic silver. The rate of decomposition increases with increasing concentration of dissolved silver oxide, temperature and the presence of various kinds of impurities.
The precipitated silver black has tremendous diffusion ability, which results in roughening of the primary ultra fine porous structure, decreased surface area of the catalyst, and correspondingly, reduced electrode performance.
While the dissolution of silver may be inhibited or diminished by deliberate, continuously maintained polarization of the electrochemical power sources having silver-catalyzed or bulk-silver-alloy-catalyzed air or oxygen cathodes, this is highly disadvantageous, and in many cases, practically impossible.
There is therefore a recognized need for, and it would be highly advantageous to have, a silver material, modified chemically, that inherently is stable and resistant to dissolution.